Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. The engine knock may be addressed by retarding spark timing; however, significant spark retard can reduce fuel economy and limit maximum torque. Knock is especially problematic under boosted conditions due to high charge temperatures.
One method to reduce charge temperature and therefore knock, is via blowthrough wherein boosted intake air is blown through the combustion chamber to the exhaust during a positive valve overlap phase.
Another method to suppress knock is by diluting intake air with cooled exhaust gas recirculation (EGR). An example approach of controlling the flow of exhaust gases for EGR is shown by Roth (U.S. Pat. No. 8,495,992) wherein a split exhaust system separates exhaust gases exiting the combustion chamber during blowdown and scavenging phases. Exhaust gases from the blowdown phase are distributed either to the turbine in a turbocharger system or to an EGR system which directs cooled EGR gases to the intake manifold or upstream of the compressor in a turbocharger. Likewise, exhaust gases from the scavenging phase are conveyed to either an emission control device or to an EGR system which delivers cooled gases to the intake manifold or upstream of the compressor. Intake and exhaust valve timings are controlled to regulate the amount of exhaust gases flowing to the turbocharger and/or EGR based on engine operating conditions.
The inventors herein have identified potential issues, including issues with the above approaches to addressing knock limits. For example, an EGR throttle may be placed in the intake, upstream of the compressor, to enhance EGR flow at low backpressure which can make the turbocharger more sensitive to surge and increase pumping losses. Further, a separate EGR cooler may be used to cool the hot exhaust gases before they can be supplied to the intake manifold, thus increasing system costs and requiring packaging space. Further still, in the example where a blowthrough technique is used to reduce knock, additional fuel injected to bring exhaust gases to a stoichiometric ratio can cause over-temperature of the catalyst and affect emissions while increasing fuel consumption. An additional limitation of the blowthrough technique is its restricted use for low engine speeds when compressor outlet pressure is higher than pre-turbine exhaust pressure.
The inventors herein have recognized the above issues and identified approaches to at least partly address the issues. In one example approach, a method for a turbocharged engine with a split exhaust manifold system is provided. The method comprises flowing exhaust through a first exhaust valve to a turbine of the turbocharger, flowing exhaust via a second exhaust valve to upstream of an emission control device and flowing low pressure EGR and blowthrough air via a third valve to upstream of a turbo-compressor into the compressor inlet within a common engine cycle combustion event. By using appropriate valve timing controls, a combination of EGR and blowthrough air techniques can be used to reduce combustion temperatures and thus, mitigate engine knock.
For example, during a combustion cycle of one cylinder of a turbocharged engine, a first blowdown exhaust portion may be directed to the turbine through a first exhaust valve which may open before bottom dead center (BDC) position of the piston to allow 75-80% of the combusted gases to exit. A second exhaust portion may be routed to an emission control device via a second exhaust valve which opens halfway through the exhaust stroke to drain 10-15% of the remaining exhaust gases, termed the “scavenging” portion. The first and second exhaust valves may close before the piston reaches top dead center (TDC) position leaving a portion (˜5-10%) of exhaust gases within the cylinder to be evacuated by a third valve coupled to the compressor inlet. The third valve, also known as a “compressor inlet valve”, may be opened towards the end of an exhaust stroke, for e.g., before TDC, and closed well past the onset of an intake stroke, for e.g., well after TDC. Consequently, the compressor inlet valve may be open at the same time that one or more intake valves are opened to admit fresh air into the cylinder. Thus, residual exhaust gases within the cylinder towards the end of the exhaust stroke may be flushed out along with fresh blowthrough and transferred to the inlet of the compressor via the compressor inlet valve.
In this way, knock can be reduced by combining blowthrough and EGR in one flow path. By allowing fresh intake air to blow through any residual hot exhaust gases in the cylinder clearance volume, the combustion chamber may be cooled. The mix of exhaust gases and blowthrough air exiting the chamber may be combined with additional fresh air at the compressor, cooled in a charge air cooler (CAC) and eventually recirculated in the engine as EGR to further reduce knock. By using the CAC to cool the residual exhaust gases along with fresh compressed air, a separate EGR cooler may not be required. Further, the EGR throttle may be dispensed with by coupling the compressor inlet valve to the compressor, whereby the mix of exhaust and blowthrough air is drawn into the low pressure inlet of the compressor through the cylinder from a high pressure intake manifold. Since blowthrough air and residual exhaust gases are directed to a compressor inlet at lower pressure, blowthrough may be possible over a greater range of engine speeds. Additionally, exhaust pumping losses encountered in a traditional design, where all exhaust flows into a high pressure turbine inlet, may be reduced. Moreover, since the blowthrough air is not directed to an emission control device, maintaining stoichiometric ratio in the exhaust with an injection of extra fuel may not be required. Overall, a turbocharged engine can be operated with less spark retard from maximum torque.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.